Six men are seriously ill in a British hospital after taking part in a clinical trial to treat chronic inflammatory conditions and leukaemia.
The men were admitted to the intensive care unit at Northwick Park hospital, north-west London, from an independent medical research unit on the hospital campus after reacting badly to the anti-inflammatory drug.
Parexel, the clinical research company running the trial, said it had operated within regulatory guidelines and that such adverse reactions to drugs were extremely rare.
The UK medicines watchdog, the Medicines and Healthcare products Regulatory Agency (MHRA), immediately suspended the trial and has sent inspectors to the research unit to carry out an investigation.
The MHRA is working with the North West London Strategic Health Authority, the Department of Health and the Metropolitan Police to determine the cause of the men's ill-health.
Professor Kent Woods, Chief Executive Officer at MHRA, said: "Our immediate priority has been to ensure that no further patients are harmed. We will now undertake an exhaustive investigation to determine the cause and ensure all appropriate actions are taken."
Eight men were taking part in the trial but two were given a placebo and were unharmed. Researchers running the trial have assured the MHRA that no-one other than the six men affected by the drugs has been given the product.
The organisation has also notified other European regulatory bodies of the problem. It is thought one other European country is currently carrying out trials on the drug.
The men, who volunteered to take part in the first stages of the clinical trial, are receiving specialist treatment in the hospital's critical care unit.
A spokesman for the hospital trust denied reports that the men were Australian but refused to comment further on the patients.